Ignores Python 2 import_recipe() warnings

Python 2 raises warnings on `imp.load_source()` when the module name
contains dots. Ignoring this warning makes p4a output/logs more
readable.
Note that for some reason the regression test will only catch the
regression if ran isolated, e.g. via:
```sh
tox -e py27 -- tests/test_recipe.py::TestRecipe::test_import_recipe
```

pythonforandroid/recipe.py

@@ -24,8 +24,16 @@
 # this import is necessary to keep imp.load_source from complaining :)
 if PY2:
     import imp
-    import_recipe = imp.load_source
+    import warnings
+
+    def import_recipe(module, filename):
+        with warnings.catch_warnings():
+            # ignores warnings raised by hierarchical module names
+            # (names containing dots) on Python 2
+            warnings.simplefilter("ignore", RuntimeWarning)
+            return imp.load_source(module, filename)
 else:
+    # Python 3.5+
     import importlib.util
     if hasattr(importlib.util, 'module_from_spec'):
         def import_recipe(module, filename):
@@ -34,6 +42,7 @@ def import_recipe(module, filename):
             spec.loader.exec_module(mod)
             return mod
     else:
+        # Python 3.3 and 3.4:
         from importlib.machinery import SourceFileLoader
 
         def import_recipe(module, filename):

tests/test_recipe.py

@@ -1,7 +1,9 @@
+import os
 import types
 import unittest
+import warnings
 from pythonforandroid.build import Context
-from pythonforandroid.recipe import Recipe
+from pythonforandroid.recipe import Recipe, import_recipe
 
 
 class TestRecipe(unittest.TestCase):
@@ -41,3 +43,18 @@ def test_get_recipe(self):
             Recipe.get_recipe(recipe_name, ctx)
         self.assertEqual(
             e.exception.args[0], 'Recipe does not exist: {}'.format(recipe_name))
+
+    def test_import_recipe(self):
+        """
+        Verifies we can dynamically import a recipe without warnings.
+        """
+        p4a_root_dir = os.path.dirname(os.path.dirname(__file__))
+        name = 'pythonforandroid.recipes.python3'
+        pathname = os.path.join(
+            *([p4a_root_dir] + name.split('.') + ['__init__.py'])
+        )
+        with warnings.catch_warnings(record=True) as recorded_warnings:
+            warnings.simplefilter("always")
+            module = import_recipe(name, pathname)
+        assert module is not None
+        assert recorded_warnings == []